1. Field of Invention
This application relates generally to high frequency measurements and more specifically to calibration standards.
2. Discussion of Related Art
In many applications it is desirable to measure the performance characteristics of electronic components. A network analyzer is a piece of test equipment often used for this purpose.
For the measurements made by a network analyzer to accurately reflect the performance of an electronic component, the network analyzer must be calibrated. Calibration is often achieved by attaching devices of known electrical properties, called standards, to the terminals of the network analyzer. By comparing the measurements made by the network analyzer on the known standards to the actual value of the standards, errors in the measurement process can be identified. These errors can be used to compute an adjustment that is applied to each measurement made with the network analyzer.
There are two popular methods for calibrating a network analyzer. One is called the SOLT (Short Open Load Through) method. This approach involves alternatively connecting a calibration standard to each of the terminals that represents a short to ground, an open circuit and a load of known impedance, often a fifty ohm load. In addition, the two ports of the network analyzer are connected together to make the “through” measurement. Taking measurements on these combination of reference standards is sufficient to allow computation of correction factors needed to calibrate the network analyzer.
These measurements are taken at multiple frequencies over the operating range of the network analyzer so that correction values are available for all the frequencies in the operating range. In practice, it is difficult to provide a standard that presents a fifty ohm load across the range of frequencies at which the network analyzer might operate.
The TRL (Through Reflect Line) calibration approach is often used for wide bandwidth or high frequency measurements because it does not require a load standard. The TRL calibration method requires three calibration standards. A “through” standard can be similar to one used in the SOLT approach. Likewise, the “reflect” standard can be the same as the “short” standard used as in the SOLT approach. The line standard is similar to the through standard, but an additional length of line is placed between the two ports of the network analyzer. The characteristics of the line standard depend on the length of line added and the frequency at which measurements are made. Accordingly, multiple line standards are often provided, with each line used to make calibration measurements over a specific range of frequencies (i.e., a specific bandwidth). Measurements taken on the through, reflect and one or more line standards allow computation of correction factors to calibrate the network analyzer.
Calibration in this fashion can compensate for any linear errors in the measurement equipment that are introduced in the signal path that is present when the calibration standards are connected to the network analyzer. The end point in the signal path that is used to connect to the calibration standards is sometimes called the “calibration plane.” If additional connectors or circuitry is added beyond the calibration plane while making measurements on a device under test, any errors introduced by these added components are not removed by the calibration process.
To remove errors introduced in the signal path between the calibration plane and the device under test, a process sometimes referred to as “de-embedding” is used. De-embedding involves developing a mathematical model of the components added between the calibration plane and the device under test. Correction factors that compensate out the effect of any such added components are then mathematically computed from the model. These correction factors are applied to any measurements taken by the network analyzer to “de-embed,” or mathematically remove, any effect of the components between the calibration plane and the device under test.
An alternative approach that combines calibration and de-embedding in one step is called “calibration through the use of equivalent fixtures.” In this approach, calibration standards are mounted in fixtures that are as similar as possible to the fixture used to mount the device under test. When the network analyzer is connected to these standards for calibration measurements, the signal path to the calibration standards has the same properties as the signal path to the device under test. Correction factors computed from measurements made with these calibration standards should also remove any errors introduced in the signal path to the device under test. The calibration plane is effectively at the device under test.
FIG. 1 shows an example of a test set-up that might be used to make performance measurements on a device under test, such as an electrical connector 110. The device under test is connected to a printed circuit board 100. Coaxial connectors such as connector 120 are attached to the board. Traces 130 within printed circuit board 100 connect the co-axial connectors 120 to connector 110.
To measure the performance of electrical connector 110, a network analyzer can be connected to coaxial connectors such as 120. In use, cables running from a network analyzer are connected to the co-axial connectors such as 120.
Before making measurements, calibration standards can be connected to the ends of those same cables. To make calibration measurements using an equivalent fixture approach, reference standards are included on test board 100. These reference standards are connected through traces to coaxial connectors that match, as closely as possible, the signal paths to the device under test. For example, coaxial connectors 140 and 142 are shown joined together by trace 144. Trace 144 might serve as a “through” reference standard and will be twice the length of trace 130. Trace 144 introduces errors that approximate the errors introduced when a signal traverses one of the traces 130 from coaxial connector 120 to a connector 110 and returns back to the network analyzer through another trace. Longer traces similarly configured can provide a “line” standard used for a TRL calibration.
A short or reflect standard might also be incorporated onto printed circuit board 100. FIG. 1 shows a coaxial connector 146 coupled through trace 148 to via hole 150. If via hole 150 is plated through, it will short trace 148 to the ground planes within board 100. Accordingly, it will act as a short circuit to the signal applied through coaxial connector 146.
FIG. 1 shows traces such as 130 as lines on the surface of printed circuit board 100. Art work on the surface of a printed circuit board, such as 100, is often used to make a visually perceptible representation of a trace within the circuit board. However, the actual electrical signals are carried on thin conductive strips, or traces, embedded within the board 100. These traces run between parallel ground planes within circuit board 100, forming stripline transmission lines to carry the signals. The artwork depicted on the surface of the board is intended as an aid in illustrating the underlying structure.
We have recognized that use of a plated through hole such as via 150 does not act as a true short circuit. In particular, we have noted that the plated through hole creates a frequency dependent reactive load, particularly at high frequencies. The problem is most noticeable at frequencies greater than 3 GHz.